Browsers display web pages by interpreting Hypertext Markup Language (HTML), which includes sets of tags nested within other sets of tags around content to render. To assist the authors and designers in creating such web pages, a recent development is the use of cascading style sheets, via which the author or designer may globally specify styles for selected page elements, including appearance information such as color and point sizes of text, page margins, and leading (spacing between lines). A style sheet is essentially a template that controls the formatting of HTML tags on a page, and can be linked to a page or embedded in a page. Style sheets separate the formatting information from the actual content on HTML pages, whereby it has become easier for authors and designers to design and revise pages.
An improvement to how a web page may be made to appear to a viewer is Dynamic HTML (DHTML), which enhances the appearance of a page's elements by adding effects thereto. Dynamic HTML (DHTML) also allows an HTML document to become interactive, which is accomplished by allowing HTML elements or tags within the HTML document to be programmable, such that script can change the content and attributes of the HTML page. For example, a script on the page may allow a list to be expanded or collapsed in response to a mouse click, without needing to retrieve a different HTML document from the server. The script resides on the same page as the other elements, even when the script is obtained from a separate file, since that script is particular to the page and is essentially imported into the page when the HTML document is loaded.
In essence, three job disciplines are involved in creating a DHTML page, i.e., a content provider, designer, and engineer. The content provider is the author of the content of the document, while the designer decides on the look and feel of the document, and typically adds any formatting information. The engineer adds the dynamic functionality using script. In many situations, these jobs are not performed by one highly-skilled person, but by three separate individuals of different skill levels, as many authors and designers are uncomfortable with programming. This, along with the fact that the script resides on the same page as the other elements, makes DHTML expensive and fragile to use in web documents. More particularly, as the engineer applies one or more scripts to every page, the process becomes expensive and error prone, as subsequent content changes by the author or designer may affect the script. At the same time, the engineer may affect the layout or content of the document when adding the script to the page. Even for those authors and designers who are comfortable with programming concepts and with using script to add dynamic functionality to a document, having the script and content in the same file makes the page creation and maintenance processes relatively slow and difficult to manage.